1. Field of the Invention
The present invention relates to thermally reworkable compositions and, in particular, to thermally reworkable epoxy compositions which are adapted for use as flip-chip underfill encapsulants.
2. Description of the Related Art
In the field of electronic packaging and, in particular, the field of integrated circuit (IC) chip interconnection, the desirability of incorporating high input/output (I/O) capability and short IC interconnects, among others, typically has led to the adoption of the flip-chip technique of IC chip interconnection. Generally, the flip-chip technique involves electrically interconnecting an IC chip and a substrate with the use of solder joints which are disposed between the IC chip and the substrate.
As initially practiced, the flip-chip technique oftentimes utilized relatively high cost materials, such as high lead solder and ceramic substrate. However, the desire to reduce costs has prompted the use of less expensive materials, such as flip-chip on board (FCOB), which typically utilizes eutectic solder and organic printed wiring board (PWB). While reducing material costs, the use of FCOB packaged systems has accentuated the problem of coefficient of thermal expansion (CTE) mismatch between the IC chip and the organic substrate of the FCOB, particularly when large IC chips and fine pitch, low profile solder joints are utilized. Due to the CTE mismatch between silicon IC chips (2.5 ppm/.degree. C.) and organic substrates, i.e., FR-4 PWB (18-24 ppm/.degree. C.), temperature cycle excursions experienced by the FCOB generate tremendous thermo-mechanical stress at the solder joints and, subsequently, can result in performance degradation of the packaged system.
It is also known in the prior art to fill the spaces or gaps remaining between an IC chip and substrate which are not occupied by solder with an underfill composition or encapsulant. The encapsulant is an adhesive, such as a resin, that serves to reinforce the physical and mechanical properties of the solder joints between the IC chip and the substrate. The encapsulant typically not only provides fatigue life enhancement of a packaged system, but also provides corrosion protection to the IC chip by sealing the electrical interconnections of the IC chip from moisture, oftentimes resulting in an improvement in fatigue life of ten to over one hundred fold, as compared to an un-encapsulated packaged system.
Heretofore, cycloaliphatic epoxies, typically combined with organic acid anhydrides as a hardener, have been used in flip-chip packaged systems as an encapsulant. This is primarily due to the low viscosity of cycloaliphatic epoxies prior to curing, as well as their acceptable adhesion properties after curing. Additionally, silica has been utilized as a filler in these encapsulant formulations, i.e., up to 70% (by weight), in order to lower the CTE of the epoxy resin. By way of example, the material properties represented in Table 1 typically are exhibited by a prior art epoxy encapsulant composition.
TABLE 1 Typical Material Properties of Epoxy Encapsulants Solids Content 100% Form Single component, pre-mixed Coefficient of Thermal Expansion 22-27 ppm/.degree. C. (.alpha..sub.1) Tg &gt;125.degree. C. Cure Temperature &lt;165.degree. C. Cure Time &lt;30 min. Working Life (@ 25.degree. C., visc. double) &gt;16 hrs. Viscosity (@ 25.degree. C.) &lt;20 kcps Filler Size 95% &lt; 15 .mu.m Filler Content &lt;70 wt % Alpha Particle Emission &lt;0.005 counts/cm.sup.2 /hr. Hardness (Shore D) &gt;85 Modulus 6-8 GPa Fracture Toughness &gt;1.3 Mpa-m.sup.1/2 Volume Resistivity (@ 25.degree. C.) &gt;10.sup.13 ohm-cm Dielectric Constant (@ 25.degree. C.) &lt;4.0 Dissipation Factor (@ 25.degree. C., 1 khz) &lt;0.005 Extractable Ions (e.g. Cl, Na, K, Fe, &lt;20 ppm total etc.) Moisture Absorption (8 hrs. boiling &lt;0.25% water)
Although the aforementioned epoxy encapsulants typically possess suitable material properties for use in flip-chip packaged systems, their intractability after curing poses a problem. In particular, once an epoxy encapsulant has cured between an IC chip and a substrate, for instance, it is extremely difficult to rework the epoxy encapsulant, thereby allowing removal of the IC chip from the substrate. Therefore, it has heretofore been desirable to form encapsulants which are reworkable under certain conditions. Several alternative prior art approaches for addressing reworkability are discussed hereinafter.
Development of reworkable encapsulants generally can be classified in two categories: chemically reworkable encapsulants and thermally reworkable encapsulants. In the category of chemically reworkable encapsulants, U.S. Pat. No. 5,560,934, issued to Afzali-Ardakani et al., discloses epoxy compositions that are soluble in an organic acid after curing. Utilizing relatively strong chemicals such as acids (or bases) during reworking, however, oftentimes leads to a messy, time-consuming rework process. Additionally, it has been found that the use of chemicals during the rework process typically makes localized repair of a packaged system difficult and, sometimes, impossible. Therefore, it is believed that thermally reworkable materials offer the possibility of a relatively quick, clean, and localized rework process.
For example, U.S. Pat. No. 5,659,203, issued to Call et al., discloses a reworkable flip-chip module utilizing a specially defined thermoplastic resin as an encapsulant. The thermoplastic resin, such as polysulfone, polyetherimide, etc., possesses a high glass transition temperature (Tg), e.g., 120.degree. C.&lt;Tg&lt;220.degree. C., and must be either dissolved in a solvent or heated above its melting point during the encapsulation process. Therefore, use of these thermoplastic resins as encapsulants for FCOB applications may be undesirable, since such applications typically require an encapsulant which is free of solvent and in liquid form during the encapsulation process, and typically require keeping the packaged system at lower temperatures in order to maintain the integrity of the eutectic solder which is utilized with the organic PWB.
Additionally, U.S. Pat No. 5,371,328, issued to Gutierrez et al., discloses a reworkable flip-chip type of circuit module using a non-stick release coating on all surfaces intermediate of the chip and the substrate. The use of a release coating, however, potentially reduces the strength of the adhesions at all of the material interfaces, including the encapsulant to chip and the encapsulant to substrate interfaces. Since it is known in the prior art that the adhesions of these interfaces are critical to the reliability of the module, the introduction of the non-stick release coating may affect the reliability of the circuit module.
Therefore, there is a need to provide an improved encapsulant which addresses these and other shortcomings of the prior art.